The group claiming responsibility for the attack on a nairobi mall is no stranger to u.S. Anti-terrorism officials. They've been recruiting followers all over the word including here in the united states. Videos with the potential american jihadi in mind are all part of the cell. Abc's brian ross investigates how they're doing it. Reporter: After four days of terror, unrelenting terror -- Reporter: Kenyan troops were able to give the thumbs up, the westgate shopping mall in nigh ronigh -- nairobi had been secured from the terrorists. At a great price, the president said. I report with great sadness that 61 civilians lost their lives in the attack. Six security officers, also made the ultimate sacrifice to defeat the criminals. Reporter: Survivors tell stories of heroism. A new video showed this man, crawling to help rescue a mother and her two children. Then running in a crouch to safety. The terrorists seen in this photo challenged hostages to recite the koran saying they only wanted to kill non-muslims. This woman told abc news correspondent she tried to memorize a muslim prayer sent by a relative in hiding. I was able to mem rye orize it in case they found me. Reporter: Five terrorists were killed during the siege with the president saying attackers and civilians are buried under the rubble caused by explosion. Three floors of the westgate mall collapsed and there are several bodies still trapped in the rubble including the terrorists. Reporter: While the terrorists were ultimately defeated, the protracted worldwide attention on the attack may have been all they were after in the first place. This is a major success for them. This was a mission to kill and maim at a very high level. And to receive the kind of attention that it has done. Reporter: The group claiming responsibility came across the border into kenya from neighboring somalia. It is called al-shabaab, arabic for "the youth or the boys." Started as a nationalistic guerrilla group, al-shabaab pledged allegiance to al qaeda and one of its most potent affiliates in the world. The fact they separated muslim and non-muslim, it is not madness, ideological terrorism. Reporter: Terrorism carried out with 21st century tools of social media. Throughout the siege, they sent audio and written messages to reporters via twitter. A warning in arabic to day that kenya should pull its troops out of somalia or else. You have seen what you will reap. Which is only the beginning. This follow-up written message in english. Rest assured, kenyans are in for a big surprise. Is that the face of terrorism in 2013? Actually, social media. Youtube videos, tweets are part of terrorist organizations propaganda and part of, their operations. The former fbi agent -- awe when they do something like this they want a lot of people watching. Reporter: The al-shabaab unit turns out videos, many aimed at americans featuring american recruits like this young man who supposedly came from minneapolis but was killed in fighting after recording this tape. If you guys only knew how much fun we have over here. This is the real disneyland. Come here, join us. Reporter: Powerful word of mouth according to london's royal united services institute. From the group's perspective, these individual are useful they help project their ideology on one hand and help bring funding and bring recruits. Reporter: There have been more than 50 americans who have signed on as recruits to al shabaab according to u.S. Officials. And the twin cities of minneapolis-st. Paul have produced more than 20 of them. I think it certainly is surprising any time you find out middle america has become a hotbed for terrorist recruiting. Reporter: Officials say most of the twin cities recruits came from neighborhoods with somali refugees, resettled by the UNITED STATES IN THE 1990s WHERE Leaders say many young men remained vulnerable and under recruitment. 15 days ago, two more young men disappeared. It is happening. Reporter: One of the recruits to al shabaab from minneapolis later killed in somalia. I know young kids who are very vulnerable right now. I know recruits are talking to every day. Taking them to dinner. Taking them this. Engaging them. Playing. From minneapolis, san diego to maine. Prosecutors charged more than a young recruits supporting terrorism including some recruited to go to somalia by a man working as a janitor at a minneapolis mosque, now convicted. He was involved in assisting young men who were ultimately sent from minnesota to -- somalia to join al shabaab. He traveled to somalia and provided money they used to purchase weapons. Community leaders have deplored al shabaab's efforts to target young men. We call on youth to shun, reject, to not be lulled into extremist groups like al shabaab. The fbi noknows of no credible threats. As concerns are raised that al shabaab's recruits could bring home what they learned over there. What happened in nairobi could happen in any mall in the united states. The mall had more security than most malls in the united states. The fbi and homeland security's role is to keep people like this out of the united states. But once they get in they could do this kind of attack. Along with the five dead terrorists, 11 suspects are in custody tonight. Able. The kenyan president said his government does not yet know if any of the american recruits took part in the attack on the west gate mall.

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